In reality, however, small amount of modal delay always remains due to small variations in fiber fabrication. It is theoretically possible to eliminate the modal delay in a multimode fiber by carefully tuning the graded index profile. A single-mode fiber has a relatively smaller core size compared to a multimode fiber, and only one propagation mode is allowed (see Figure 4). If only one mode is allowed to propagate, the fiber is called a single-mode fiber. If more than one mode is allowed to propagate in a fiber at a certain wavelength, the fiber is called a multimode fiber (at that wavelength). The number of allowed optical paths in an optical fiber, called modes, depends on the refractive-index profile. The difference in the propagation velocity, therefore, becomes much smaller and degradation in the pulse shape is reduced. The path length is still longer for the blue ray however the blue ray travels at a faster speed than the red ray as the refractive index of the outer side of the core is lower than that of the center. Figure 3(b) shows that, light launched at a small angle (red ray) mainly travels near the center of the core, and light launched at a large angle (blue ray) travels at the outer side of the core. GI fiber was introduced in order to overcome this limitation of SI fiber. This is problematic for high-speed optical communication as the incident pulse shape is degraded at the output. Figure 3(a) intuitively explains that the light launched at a larger incident angle (blue ray in the figure) propagates at a slower velocity, as the path length is longer. When an optical pulse is launched into a SI fiber, the propagation velocity of the pulse is highly dependent on the incident angle. The largest difference between these two types of optical fibers lies in the ability to maintain the pulse shape after propagation.įigure 3: Light guidance in (1) step-index and (2) graded-index fiber. The core of a SI fiber has a uniformly raised refractive index profile, and that of a GI fiber has a continuously raised refractive index profile.
There are two major types of refractive-index profiles – step index (SI) and graded index (GI) – and they are schematically shown in Figure 3. This equation suggests an optical fiber with a higher NA allows the launch of light with a larger launch angle, therefore enables the launch of higher power density into the core. If n core > n clad and θ is larger than the critical angle θ c which is provided by The refractive indices of the core and cladding are n core and n clad, and a ray is travelling in the core with an angle θ. The refractive-index modification enables the transverse confinement of light in the core, and light is guided longitudinally in the core.įigure 2: Schematic of optical fiber, and a ray confined in the core by TIR.įigure 2 shows schematically, using ray optics, the light guided in the core by TIR.
A core is embedded in a cladding and modifies the refractive index in the transverse (x and y) direction, and the modification is uniform in the longitudinal (z) direction. Figure 2 shows schematically the most basic form of a modern optical fiber. TIR takes place at any interface between two materials with different refractive indices, not only at the interface of water and air. This phenomenon highlights two very important features of optical fiber – long and bendable.įigure 1: Light guidance in fountain by TIR. The history of optical fibers is believed to date back to mid 1800s, when an Irish physicist John Tyndall demonstrated light guidance in a fountain (see Figure 1). Light is guided in a stream of water by total internal reflection (TIR) even though the water stream is bent by gravity. Unlike other form of waveguides, optical fibers can be manufactured long and bendable, and thus are primarily used for a transmission media for optical fiber communication. Optical fibers are one of the most widely used form of optical waveguide.